Development of disease resistance among Chesapeake Bay
oysters calls for a shift in oyster-restoration strategies within the Bay and
its tributaries. That’s according to a new study by researchers at the Virginia
Institute of Marine Science.

Carnegie, a Research Assistant Professor in the Shellfish
Pathology Lab at VIMS, says “Our results point to substantial reproduction by disease-resistant
oysters in the high-salinity areas where the parasite causing MSX thrives. We
thus argue that reefs in areas of higher salinity should be the focus of conservation
and restoration efforts, not just those in disease-free lower salinity areas.”

MSX is caused by the single-celled
parasite Haplosporidium nelsoni. It
first appeared in Chesapeake Bay in 1959, combining with overharvesting, declines
in water quality, and a second parasitic oyster disease known as Dermo to push
the Bay’s oyster population to one percent of historical levels. Both MSX and
Dermo favor the saltier waters of the Bay’s main stem, decreasing in prevalence
as one moves upriver into the fresher waters of Bay tributaries, which form a
safe haven from the parasites and diseases.

Restoration strategies

To date, restoration strategies have rested on the idea of
protecting these “low-salinity refugia” as sources of larvae for replenishment
of disease-ravaged populations in saltier areas of the Bay. These strategies
are based on the high levels of mortality traditionally seen among oyster
populations in saltier waters (initially more than 90 per cent), and computer
models showing that tidal currents can indeed carry oyster larvae downriver from
fresher to saltier areas.

Carnegie says “there’s been a lot of attention given to
these up-river, low salinity refugia. They’ve been viewed as the key source
reefs that are exporting larvae into the higher salinity waters.” Disease-ravaged
reefs in higher salinities have been valued primarily for their fishery, which
has sought to harvest doomed oysters before they succumbed.

Carnegie and Burreson’s research, however, paints a
different picture. They’ve found clear evidence that oysters in the Bay’s
saltier waters are developing resistance to both MSX—the focus of their paper—and
Dermo, despite the increasing prevalence in the Bay of the parasites responsible
for the two diseases. This is only possible through reproduction by resistant oysters
in high-disease areas, thus their call for a focusing of restoration efforts onto
these disease-resistant areas and populations.

Carnegie says "We
basically need to confront the diseases head-on where they are most active,
rather than avoiding them by working in low salinities. It’s in the
high-disease areas that resistance is developing most rapidly, so restoration
efforts should be focused there."

Recent restoration initiatives in Virginia have included the
designation of numerous sanctuary reefs in higher salinities and a rotational harvest
scheme in the lower Rappahannock River, but these have been controversial. “Harvesters
have viewed such efforts with skepticism,” says Carnegie, “because the protected
oysters would likely be lost to diseases. Yet our results suggest that strategies
like these that increase the number of resistant oysters are precisely the right
approach.”

In fact, Carnegie and Burreson point out that restoration
efforts focused on low-salinity refugia may actually be counter productive.
Carnegie says “our study makes very clear what happens when oyster larvae from
the low-salinity refugia settle in high salinity waters—they are removed
quickly by the parasites because they are produced by genetically naïve parent
oysters that haven’t been selected for disease resistance.” Genetic mingling of
these susceptible oysters with their disease-resistant counterparts may be
acting to slow the spread of disease resistance through the Chesapeake Bay
oyster population.

"Spring Imports” project

Discovery of disease resistance among the Bay’s oysters is
based on research that began at VIMS in 1960 with the “Spring Imports” project.
Burreson says this long-term study “clearly shows increased resistance to MSX
in response to increased disease pressure.” Carnegie adds “decreased disease in
the wild despite favorable conditions for the parasites is a clear sign of
increasing resistance among our native oysters due to long-term exposure.”

The pair’s research was
supported in part by the A. Marshall Acuff, Sr. Memorial Endowment to the VIMS
Foundation, which has supported oyster disease research at VIMS for almost 30
years. Carnegie says, “Our team really appreciates the flexibility Acuff
support gives our program—the ability to rapidly respond to emerging questions,
pursue new directions, and build the foundation for larger, extramurally funded
projects.” Acuff support has played a key role in the renewed commitment
to native oyster restoration, and has leveraged federal funds and created jobs
for Virginia citizens.